You know, I'm a drummer, and despite all the snarky jokes about drummers can't read any music that's not on a straight line, I do respect my other musical peers need for reading key sigs, and distinguishing between minor, and Major sigs.  It's kind of like ignoring the time sig 2/4, and just using 4/4, since all you need to do is add 2 of them, and you've got 4/4.  So, why does Sonar ignore minor key sigs.  Or, at the very least, allow a user to add them to an .INI file if, and when they need them?  That's all I got to say 'bout that!
 
KC

It's usual to use the key signature of the minor key's relative major e.g. G major for E minor etc.

I thought it did allow for minor keys to be chosen.
 
However all you have to do is is count up to the third note of the minor scale you are using and you will have the relative major.
 
Example: A minor... A, B, C.
 
Use the key sig for C Major.

jb101 is right to help you the relative major is the 3rd note in your minor scale.... or 3 half steps above your root note.
there is more than one minor scale but this will hold for the normal minor scale.
 
If you want to use a dorian minor then the key sig would be down 1 step (2 half steps) from your root. i.e. d-dorian is the same as the key of c and a dorian is teh key of g and so forth.
 
HTH

Yup, major and minor keys share key signatures (Cmaj & Amin, Fmaj & Dmin etc.)
 
The only way to differentiate them is to check the score out and work out what the implied root is (e.g. from the melody or dominant->root cadences etc.)  Or somebody might helpfully write 'Ab minor' or something on the score.
 
Also, isn't all music written on straight lines? It'd all be a bit confusing if not :^)  I find it confusing enough as it is...

Well I was attempting to keep it simple but just for reference these are all the modes/scales in relation to the Natural Major (Ionian) scale:
 
I: Ionian (Natural Major)
II: Dorian (minor)
III: Phrygian (minor)
IV: Lydian (Major)
V: Mixolydian (Major/Dominant)
VI: Aeolian (Natural minor)*
VII: Locrian (minor/half diminished)
 
Typically when someone simply says Major or minor they are referring to the Natural Major (Ionian) and the Natural minor (Aeolian).
 
All of the modes/scales listed use the same key sig.
 
*The Melodic and Harmonic minor scales appear on the 6th step and are simply variations of the Natural minor scale (Aeolian). They will use the same key signature as all of the above however the altered notes will be shown as accidentals (sharps/flats) throughout the sheet music. Also when you use the Harmonic/Melodic minors you create a whole new set of modes that have different names and different people use different names for them (like jazz guys will call them something different than classical guys and even within the styles there is still discrepancies). I usually just call them by their natural names with a qualifier like "b 6th".

Some people may find it easier to relate to by explaining that whatever major key you have has a relative minor. If you start up the major scale your relative minor is ALWAYS the sixth step. EX. Key=Cmajor, Major Scale of C= C,D,E,F,G,A,B , Relative Minor=sixth step or A. C major and A minor are the same signature on the staff. The above post is correct about starting from the A not in the minor scale as well. Just depends on how you look at it.
Every key in theory abides by this rule. 

As far as I know, whether in Sonar or on music paper, you just identify the number of sharps or flats.  "Key" is more of an interpretation.
 
Three sharps is an absolute key signature.  Whether that represents A major, F# Minor, or both at different times in the piece, or neither one, is a matter of interpretation.  If I tell someone the song is in A major, if it is an extremely simple song with simple chords that clearly starts and ends with an A major chord, then it is pretty straightforward.  In a more complex scenario, saying, "A major" only lets the person know that there are three sharps in the key signature, and even those could change during the song.
 
In addition to major and minor, there are other scales and other key modes, so it seems simpler for Sonar just to list the number of flats or sharps and put the major key as a reference or shortcut.
 
This is somewhat like explaining left and right.  Those are not absolute values and the depend on some other reference point.

Yup (this "yup" was in reference to Sidroe's post... the rest is just bored blathering). To find the relative minor of a Major key go forward six steps in the scale or 3 steps back from the root.
 
When we talk about modes and relative Majors/minors all that is happening is we are taking the same 7 notes and changing the start/end point of the scale. There are twelve keys so that means we have twelve such patterns using the Natural Major pattern and each of those twelve has a subset of 7 possible linear 7 note arrangements.
 
It is all just a shifting grid. Once you can visualize it as such it becomes much easier to understand. To understand how keys work in relation to each other study the Circle of Fifths which may seem complex at first glance but is actually quite simple.
 
Start at C which has no sharps or flats then find the 5th note (which is G). G is now your new starting point in the CDEFGABC pattern. Sharp the 7th note (B) and you will have the key of G Major from which you can create all 7 modes of G Major. Then find the 5th note of G Major (which is D) and again sharp the 7th note. You have now created D major. Follow this pattern until every letter note is sharped. Then you have to switch over to using flat notes and the process kind of reverses and you remove flat notes to preserve the Major pattern until you return back to C.
 
You can do it in reverse starting from C and counting up to the 4th. We do not add the flats to the 7th though. We add them to the 4th. So find the fourth of C (which is F) then count up to the 4th (which is B). Flat the B. Now you have created the key of F Major. I refer to this as the Circle of Fourths and have seen it called that elsewhere as well but not very often.
 
When you do this you will notice that there are three keys in the middle that can be interpreted as either sharp or flat. They are:
 
C# = Db
F# = Gb
Cb = B
 
Usually you would use the sharp version to write in these keys but not always. Also in certain cases you will need to sharp a letter note even though the note above it is only a half step away (eg: E# = F in the key of F# Major). This is done to preserve the letter order because in diatonic scales you never use a letter note twice. You sharp or flat the letter names to create your pattern.
 
When writing the key sig onto the staff you write the sharps and flats in the order they appear within the circle of fifths or the circle of fourths. This means you start at C and add one sharp to the staff for each consecutive step on the circle of fifths and for the flat keys you start from C and add one flat to the key sig follow the circle of fourths.

King Conga
You know, I'm a drummer, and despite all the snarky jokes about drummers can't read any music that's not on a straight line, I do respect my other musical peers need for reading key sigs, and distinguishing between minor, and Major sigs.  It's kind of like ignoring the time sig 2/4, and just using 4/4, since all you need to do is add 2 of them, and you've got 4/4.  So, why does Sonar ignore minor key sigs.  Or, at the very least, allow a user to add them to an .INI file if, and when they need them?  That's all I got to say 'bout that!
 
KC

You've got a lot of excellent information in the replies, most of which will make absolutely no sense to you because there isn't a real simple answer to your question. What the key signature does is establish which notes are a half step from the next note, up or down, and which notes are a whole step (2 half steps equal 1 whole step), up or down, from the next note. So it creates a pattern of whole steps and half steps. This kind of pattern is called a diatonic scale (a scale using both whole steps and half steps). The notes you use on the staff will determine what key it is.
 
We most commonly look at a key signature and use the name of the major key associated with it, but that's just a convenience. A key signature with no flats or sharps can be C major, A Minor, or any other mode. You drummers have it easy. :-)